The present invention relates to the field of metal plating, and more specifically to novel micro-throwing alloy undercoatings and a method for improving the corrosion resistance of a ferrous metal substrate by utilization of a micro-throwing alloy as the initial layer, or undercoating, applied directly thereover.
A persistant problem in the field has been to provide ferrous metal articles which are resistant to corrosion that inherently attacks such articles in normal usage. With some ferrous metal articles, the need to provide the maximum degree of corrosion resistance is particularly great, in view of the corrosive environments in which they are utilized. For example, metal fasteners which are used extensively in automotive as well as other industrial applications are typically exposed to corrosive salts and other corrosive agents which would cause rapid deterioration, both functionally and aesthetically, of such articles. Numerous other articles made of ferrous metals must be protected or corrosion will eventually occur.
It is well known that the corrosion resistance of ferrous metal articles can be improved by applying metallic coatings, either in single or multiple layers. Such a layer or layers of metal provide either greater inherent resistance to attack by corrosive agents than the ferrous metal substrate, or they are "sacrificial" in that they are preferentially attacked by corrosive agents.
For example, it is known that improvements in the corrosion resistance of a ferrous metal substrate can be achieved by applying separate layers of copper, cadmium, zinc, nickel, tin and like metals and alloys. Organic coatings, such as paints, and dyes, and chromate films have also been used over ferrous metals to improve resistance to corrosion.
However, further improvements in the corrosion resistance of such plated or coated ferrous metal articles is still necessary, particularly in view of the present trend to use such articles over a longer period of time and with exposure to corrosive environments. Improvement is also needed in the reliability with which such multi-layer platings can be applied to ferrous metal articles.
It is particularly difficult to achieve good, reliable corrosion resistance by multi-layer platings on ferrous metal articles which have a rough, uneven surface, such as results from producing the article by heading or stamping operations. The surface of these types of articles can contain pits, laps, cracks, scratches, surface defects and other irregularities which make uniform plating onto the surface difficult. Conventional electrodeposited platings are incapable of completely and uniformly coating such defects, which produce areas of low current density.